The present invention is generally related to porous silicon substrates and, more particularly, is related to a method for photoluminescence enhancement, photoluminescence stabilization, and the metallization of porous silicon substrates.
High surface area porous silicon (PS) substrates formed in wafer scale through electrochemical (EC) etching fall into two groups. PS substrates fabricated from aqueous electrolytes consists of highly branched nonporous substrates while PS substrates fabricated from a nanoqueous electrolyte is comprised of open and accessible macroporous substrates with deep, wide, well-ordered channels.
High-surface area substrates formed in wafer scale through etching display a visible photoluminescence (PL) upon excitation (PLE) with a variety of visible and ultraviolet light sources. This room-temperature luminescence has attracted considerable attention primarily because of its potential use in the development of silicon-based optoelectronics, displays, and sensors.
Although the PL is thought to emanate from regions near the PS substrate surface, the origin of the PL is the source of some controversy as the efficiency and wavelength range of the emitted light can be affected by the physical and electronic properties of the surface, the nature of the etching solution, and the nature of the environment into which the etched sample is placed. Given this range of parameters, it is surprising that, with few exceptions, PL spectra are reported for PS substrates formed in dilute aqueous HF solutions, that have already been dried in air or more inert environments following etch and rinse treatments. These ex situ samples, while providing spectral information, do not indicate the evolution of the PS substrates, and thus, they do not indicate means by which it might be modified and enhanced during or following the etch treatment.
An existing problem in fabricating PS devices rests with establishing electrical contact to the PS substrates. Another problem with PS includes the relatively long excited-state lifetime associated with the PS substrate PL. A further problem includes the relatively low PL quantum yield and the instability of the PL from PS substrates.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.
An embodiment of the present invention provides for a post-etch treatment method of enhancing and stabilizing the PL from a PS substrate. The method includes treating the PS substrate with an aqueous hydrochloric acid (HCl(H2O)) solution and then treating the PS substrate with an alcohol. Another exemplary embodiment provides a post-etch method of enhancing and stabilizing the PL from a PS substrate, which includes treating the PS substrate with an HCl(H2O) and alcohol solution.
Still another embodiment provides a post-etch method for metallizing a PS substrate in an electroless environment. The method includes treating the PS substrate with an HCl(H2O) solution and then treating the PS substrate with an alcohol, or alternatively, treating the PS substrate with a hydrochloric acid/alcohol solution. Subsequently, the PS substrate is treated a hydrazine solution to remove fluorides from the PS substrate. Next, a metal-containing electroless solution is introduced to the PS substrate. Thereafter, the PS substrate is illuminated with a light source at wavelengths less than about 750 nanometers to cause PL of the PS substrate. Then, the metal from the metal-containing solution is induced by the PL to reduce onto the PS substrate (e.g. metallization).
A further embodiment provides for a post-etch method of enhancing PL from a PS substrate by treating the PS substrate with a dye. The dye can be selected from the group including, but not limited to 3,3-diethyloxadicarbacyamine iodide; Rhodamine dye compounds; Fluorocein; and dicyanomethylene (DCM) dye compounds.
Still a further embodiment provides for a metallized PS substrate. The PS substrate can be metallized with copper, silver, or other appropriate metal and can have a resistance of about 20-100 ohm.
Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.
One of a number of embodiments of the present invention includes the treatment of PS substrates generated in an aqueous and nonaqueous etch with an HCl(H2O) solution, which results in the stabilization and enhancement of the in situ PL of the PS substrates. More specifically, in an exemplary embodiment, in a post-etch treatment method, an HCl(H2O) solution can be used to enhance and stabilize the PL (in situ) from a PS substrate. In addition, in an other exemplary embodiment, a method of treating the PS substrate with HCl(H2O) followed by an alcohol solution (e.g. methanol or ethanol) further enhances and stabilizes the PL (in situ and ex situ) of the substrate. A non-limiting illustrative example includes PS substrates that are treated in an aqueous hydrochloric acid and water (HCl/H2O) solution and display a strongly enhanced in-situ luminescence; however, the PL decays rapidly in an ex-situ environment without treatment in alcohol, preferably a high purity alcohol. An exemplary embodiment includes treating the PS with methanol (MeOH). Further, PS substrates treated in an HCl (H2O)/ alcohol solution (of at least 0.2 molar (M)) maintain their enhancement for extended periods of time. The PS substrate may be stabilized and enhanced by the presence of a chloride ion (Cl). The treatment appears to be independent of the method of preparing the PS substrate, implying that the chloride salt treatment largely stabilizes the surface states of the photoluminescent PS substrate. This stabilization may be demonstrated by various techniques including, but not limited to the following: scanning electron micrographs (SEM), which show the profound change which accompanies the HCl treatment of the PS surface; Energy Dispersive Spectroscopy (EDS) which, reveals chloride incorporation into the PS surface at strongly photoluminescent regions; and Raman scattering, which demonstrates that the PL is correlated with the creation of amorphous structural regions. All of these testing methods indicate the manner in which the chloride salt stabilizes the PS substrate.
Another exemplary embodiment of the present invention includes treating PS substrates with a dye (e.g., 3,3xe2x80x2-diethyloxadicarbocyanine iodide (DODCI) and Rhodamine 700). In general, the dye should have negligible absorption at the wavelengths of maximum absorption for the PS substrate. After a period of aging in darkness these dye-treated PS substrates can be pumped at about 337.1 nanometers (nm) (nitrogen laser) near the maximum in the PS absorption spectrum (far from the major absorption regions of the impregnating dye). Time-dependent PL histograms indicate that the resulting PL emission rate is enhanced. The enhancement in the PL emission rate may be attributed to an interaction between the surface-bound fluorophors, which characterize PS substrates and the dye. This interaction results in the creation of a distribution of PS-dye complexes, which enhance the nominal PL emission rate from the untreated PS surface. In a preferred embodiment, the DODCI treated samples display PL that exceeds that of nominally prepared PS by a factor of five or more.
A further exemplary embodiment of the present invention includes the metallization of a PS substrate. One of the existing challenges in fabricating PS devices rests with establishing electrical contact to the PS substrate. In an exemplary embodiment, PS substrates are capable of being metallized in a controlled manner using electroless metal-coating solutions and inducing the metal to plate onto the PS substrate. An examplary embodiment includes using an electroless metal solution, which can be introduced to the PS substrate after treating the PS substrate with a hydrazine solution so that subsequently the metal can be deposited onto the PS substrate in a controlled manner. The metal-containing solution includes, but is not limited to, any one, or all, or combination of copper, silver, gold, nickel, palladium, platinum, other metals that are commonly deposited using electroless techniques. This method is capable of using the xe2x80x9clong-livedxe2x80x9d PS substrate PL to enhance reduction at the PS substrates surface. This may be accomplished by creating excited fluorophors on the PS surface to enhance interaction and reduction at the PS substrate surface. Using this method enables metals to readily deposit onto the PS substrate within PS micropores and nanopores. Further, under controlled conditions the metallization only occurs where the PS surface is illuminated with light from a light source. (e.g. Xenon (Xe) arc lamp, Helium-Neon (HeNe) laser, or other appropriate light source). Furthermore, the thickness of the metallization deposit is proportional to the time and intensity of exposure of the PS surface to the light source.
In conventional electroless metal plating, the surface is usually first coated with palladium (Pd) metal to catalyze the deposition process. For purposes of this disclosure, the addition of a catalyst to the metal plating process is considered to be operating under catalytic conditions. However, embodiments of the present invention do not require an additional catalyst, which, for purposes of this disclosure, means that the method is performed under xe2x80x9cnon-catalytic conditions.xe2x80x9d Indeed, in the method of the present invention, The illuminated PS surface itself is catalyzing the deposition. Further, localized heating is not promoting the deposition; rather the metal deposition occurs when the PS substrate is illuminated at wavelengths less than about 750 nm, consistent with its bandgap.
A further exemplary embodiment of the present invention includes the metallization of a PS substrate to produce a low electrical resistance metallized PS substrate that has a resistance from about 20 ohms to about 1000 ohms. Another embodiment includes metallized PS substrates with resistances between about 20 ohms and about 100 ohms. Still a further embodiment includes metallizated PS substrates with resistances between about 20 ohms and about 60 ohms.
As discussed above, an exemplary embodiment of the present invention includes a method and system of treating PS substrates with an HCl/(H2O) solution to enhance and stabilize the PL of the PS substrates. PS substrates treated in an HCl/(H2O) solution display a strongly enhanced in sitU PL. PS substrates treated in an HCl/(H2O) alcohol solution (e.g. at least 0.2 M) display enhanced in situ and ex situ PL and can maintain enhancement for time periods on the order of years. Another exemplary embodiment includes treating the porous silicon substrates with an HCl/(H2O) solution (e.g. at least 0.2 M) then subsequently treating the PS substrates with an alcohol. This embodiment also enhances and stabilizes the in situ and ex situ PL of the PS substrate.
More specifically, the post-etch method of enhancing and stabilizing the PL of a PS substrate includes treating the PS substrate with an HCl/(H2O) solution. The PS substrate includes, but is not limited to a microporous framework upon which is superimposed a nanoporous layer. The HCl/(H2O) solution is at least 0.2 M. In one exemplary embodiment, the HCl/(H2O) solution includes an alcohol. Alcohols that can be used include, but are not limited to, ethanol, methanol, other appropriate alcohols for treating PS substrates, and combinations thereof. In another exemplary embodiment, the PS substrate is treated with the HCl/(H2O) solution, then subsequently treated with an alcohol (e.g., ethanol, methanol, etc.) This method of treatment enhances the in situ and ex situ PL.
Chloride-ion stabilization appears independent of the method of preparing the PS substrates, implying that the chloride salt treatment largely stabilizes the surface constituency of the photoluminescent PS substrate. This can be demonstrated by scanning electron micrographs, which show the change that accompanies the HCl treatment of the PS substrate surface. Further, energy dispersive spectroscopy reveals chloride incorporation into the PS surface at strongly PL regions. Furthermore, Raman scattering demonstrates that the PS substrate PL enhancement is correlated with the creation of amorphous structural regions. In conjunction with detailed quantum-chemical modeling, time-dependent histograms obtained for the HCl-treated systems indicate that the resulting PL, initiated through the pumping of the HCl-modified surface, displays the manifestation of a significant surface interaction. This interaction might result in the formation of both chlorosilanones and chlorsilylenes. In addition, the hydrogen cation (H+) may play a role in the stabilization of the silanol-based features of the PS substrate surface both as a contribution to the flourophor formation and by decreasing the hydroxyl (OHxe2x88x92) concentration in solution.